XMP, (File Info) Custom Name Spaces, metadata in CC 2018

If I remember that correctly, the File Info dialogs have been rewritten a while ago to not use Flash any more. They should still be extensible, so instead of relying on the "advanced" tab showing up in a future release, you / your DAM vendor / your industry could develop an own tab that fits your purposes. That should also cover PS and AI. Acrobat has always used different approaches, no wonder you see differences.

XMP is also accessible by InDesign scripts so you could also show your data via a completely separate dialog.

Finally XMP can also get covered by InDesign plugins, which could add a column to the links panel, add a preflight rule, show a separate palette panel, highlight offending images in the layout, extract special fields for captions, draw a watermark and so forth. For example I have written a plugin that picks up missing fields from old IPTC metadata and other embedded resources of images, then copies it into the image's XMP as output during some PDF exports.

The "File Info" panel no longer allows me to see an "Advanced" tab. In previous versions this "Advanced" tab is where custom XMP namespaces were visible.

What was the last version of InDesign where you had an Advanced tab in File/Preferences? I don't recall it being there for awhile.

You're correct: it's no longer in InDesign but is still there in Acrobat Pro. Both programs default to "adobe:ns:meta/" namespace, but Acrobat allows you to add other namespaces.

File a bug/feature request at InDesign User Voice Adobe InDesign Feedback  Let Adobe know why you need it, how you use it in your particular workflow.

The Adobe techs there might also have a solution for you.